1. Field of the Invention
The present invention relates to a data transmission system that transmits signals by using visible light and, in particular, to an optical receiver and a visible light communication device suitable for communication using a white light emitting diode (which will be called white LED, hereinafter) including light emission by a phosphor.
2. Description of the Related Technology
In recent years, white LEDs have been developed increasingly as diverse as illuminations, lamps for cars and liquid crystal backlights. A white LED is characterized in that the speed of ON/OFF switching response is significantly higher than that of white light sources such as a fluorescent lamp. Accordingly, a visible light communication system has been proposed that provides a data transmission function to illumination light by a white LED by using white light by an LED as a data transmission medium (refer to Japanese Patent No. 3465017). In other words, the strength of emitted light by a white LED is modulated according to the transmit data, and the receiver side converts the strength of the light to an electric signal by using a photo-electric converter such as a photodiode (which will be called PD, hereinafter) to implement the data transmission.
White LEDs can be roughly categorized into three types based on the emission types as in “Shiroiro LED Shomei Shisutemu Gijutsu no Ouyou to Shorai tenbou (Applications and Future Prospects of White LED Illumination System Technology), CMC Publishing.
[1] Blue light-excited white LED: A combination of a blue LED and a phosphor that mainly emits yellow light. A YAG (yttrium aluminum garnet) based phosphor is typically placed around a blue LED in one package. In this type, the surrounding phosphor is excited by blue light output from the blue LED placed at the center, and the phosphor outputs light in a color (mainly yellow ray) in complementary color relation with blue. Mixing the yellow fluorescence by the phosphor and blue light by the blue LED provides pseudo white light.
The advantages of the blue light excited white LED may include (1) higher efficiency of energy use and higher illumination compared with the other type and (2) the inexpensive manufacturing since the construction is simple. The disadvantages, on the other hand, may include a low color rendering property. The term, “color rendering property” refers to a property how the color of an object appears under an illumination, and it is said that the closer color an object appears to the color under natural light the better the color rendering property is.
FIG. 6A shows an example of the blue light-excited white LED. As shown in FIG. 6A, a blue LED 900 is provided on the main surface of a resin case 902. A driving voltage receiving terminal (not shown) of the blue LED 900 connects to extractor electrodes 904 and 906. When driving voltage is applied to the extractor electrodes 904 and 906, blue light is output from the blue LED 900 and is partially input to a phosphor 908. Thus, the phosphor 908 is excited, and fluorescence is output. Among blue LEDs having emission characteristics with various wavelengths, one with the peak wavelength within a range of 440 to 470 nm is used here. The phosphor 908 to be used is one that emits light with a longer wavelength than the peak wavelength of the blue LED. Then, in order not to block the light emitted by the blue LED, phosphor particles 908 are placed in a radiant and thick translucent resin. FIG. 6B shows a spectrum characteristic of a blue light-excited white LED. The part surrounded by a broken line SA is a blue light part, and the part surrounded by the broken line SB is an emitted light part by a phosphor. Such light emission by a phosphor may exist on the longer wavelength side than the peak wavelength of the emitted light by a blue LED.
[2] An ultraviolet light-excited white LED is a combination of an ultraviolet LED and phosphors that emit rays in three primary colors of R, G and B (red, green and blue). The phosphors that emit rays in three primary colors of R, G and B are placed around the ultraviolet LED in one package. In this type, the surrounding phosphors are excited by ultraviolet light output from the ultraviolet LED at the center, and the rays in three primary colors of R, G and B are output from the phosphors. White light is obtained by mixing the R, C and B rays.
Among ultraviolet LEDs having emission characteristics with various wavelengths, the one with the peak wavelength in a range of about 380 to 410 nm is used here. The phosphors to be used are ones that emit light with longer wavelengths than the peak wavelength of the ultraviolet LED. Then, in order to sufficiently absorb the light emitted by the ultraviolet LED for excitation, many phosphor particles, which emit three primary color rays, are placed within a radiant and thick translucent resin.
The advantage of the ultraviolet light-excited white LED may be a good color rendering property as described above. The disadvantages on the other hand may be that (1) it is difficult to obtain a high illumination since the efficiency of energy use is lower than that of the blue light-excited white LED and (2) the driving voltage for the LED is high due to the ultraviolet light emission.
[3] A tri-color white LED is a combination of R, G and B LEDs. The structure has three types of LED of a red LED, a green LED and a blue LED in one package. In this type, white light is obtained by causing the simultaneous emission by the three primary color LEDs.
The advantage of the tri-color white LED may be a good color rendering property like the ultraviolet light-excited white LED. The disadvantage on the other hand may be that it is more expensive than other types since three types of LED are implemented in one package.
Next, characteristics of a case where the white LEDs in those types are used for data transmission are as follows:
[1] In a case where the blue light-excited white LED is used, a transmission rate of about several Mbps can be only obtained since the speed of response of light output from the phosphor is low (refer to “Kashikou Tsushin Yo LED Doraiba No Shisaku To Kashikou LED No Outou Seinou No Hyouka (Prototyping LED Driver for Visible Light Communication and Evaluating Response Performance of Visible Light LED)”, IEICE Technocal Report ICD 2005-44, Vol. 105, No. 184). FIG. 7A shows a configuration of the transceiver in the type. In FIG. 7A, the data to be transmitted is input to and undergoes predetermined modulation in a modulator 922 of a transmitter 920, and the modulation signal is supplied to a blue light-excited white LED 924. Thus, the output light of the blue light-excited white LED 924 is modulated by a modulation method such as OOK (On-Off Keying) and blinks. The blinking light after the modulation is input to and converted to an electric signal in a photo-electric converter 932 in a receiver 930, is amplified in an amplifier 934 and then is input to a demodulator 936 where the data demodulation is performed. Here, when the on/off switching of light emission is performed rapidly on the transmitter side, dull waveforms occur due to the low speed of response of light emitted from the phosphor 908, which may cause an intercode interference. In other words, as shown in FIG. 7B, the output signal SQ of the photo-electric converter 932 is dulled against the modulation signal SP by the modulator 922. This may inhibit the implementation of high speed transmission by using a blue light-excited white LED.
In order to solve the problem, a method for increasing the speed has been disclosed (refer to Japanese Patent No. 3465017 below) in which an optical component with a lower speed of response, which is output from a phosphor, is rejected by an LED light transmission color filter that transmits blue only, which is provided before the photo-electric converter. FIG. 7C shows the transceiver configuration in this case in which an LED light transmission color filter 952 is placed on the light incident side of the photo-electric converter 932 of the receiver 950. The LED light transmission color filter 952 rejects the light emitted from the phosphor with a lower speed of response in an optical signal. Thus, light by the blue LED 900 is the only input to the photo-electric converter 932, and faster data transmission than that of the configuration above can be performed as a result. However, a transmission rate of about several tens Mbps can only be obtained even by using this method. In addition, as disclosed in “Shiroiro LED No Kido Rekka No Kousatsu (Review on Brightness Deterioration of White LED)”, 2006, The Institute of Electronics, Information, Communication Engineers (IEICE), Engineering Sciences Society Conference, it has been pointed out that the data transmission quality may not be maintained due to the dispersion and secular deterioration of the emission characteristic and the uneven spatial distribution of color temperatures of the white LED.
[2] In a case where the ultraviolet light-excited white LED is used, the transmission rate may be about several Mbps for the same reason in the case using the blue light-excited white LED. In addition, it is difficult to configure a driver since the driving voltage for the LED is high.
In a case where the tri-color white LED is used, data transmission is allowed by wavelength multiplexing in which the LEDs transfer different signals, which increases the speed, with a less optical component emitted by the phosphor than that of the method above (refer to JP-A-2002-290335). However, the use of multiple LEDs may increase the costs.
Next, the quantity of light received by a PD is inversely proportional to the square of the distance if free space optical transmission is performed by a combination of a diffused light source such as an LED and a photo-electric converter such as a PD (refer to “Kukan Densou Kougaku (Spatial Transmission Optics)” Suiyosha, Chapter 6 or “An Optical Analysis of Reception Characteristic for Parallel Optical Wireless Communication System”, 2005, The Institute of Electronics, Information, Communication Engineers (IEICE), Communications Society Conference). For that reason, a larger dynamic range is required on the receiver side in order to obtain a range of length for allowing the transmission of information to some extent. For example, in IrDA (Infra-red Data Association), which is the standard for infrared communication, a dynamic range of 100 dB or larger is required on the receiver side for a range of transmission length of 1 cm to 100 cm (refer to JP-A-10-51387 or “Sekigaisen Tsushin Gijutsu (Infrared Communication Technology)”, TRICEPS, Chapter 2).
The turbulence light such as sunlight and light from a fluorescent lamp is significantly dominant in a case where free space optical transmission is performed by using light in a wavelength range of visible light as a data transmission medium. It is also difficult to optically reject the turbulence light through an optical filter to reject visible light, unlike infrared communication. For that reason, severer requirements are imposed on the receiver side. Generally, in order to obtain a larger dynamic range on the receiver side, an AGC (Automatic Cain Control) circuit may be used for the amplifier (“Sekigaisen Tsushin Gijutsu (Infrared Communication Technology)”, TRICEPS, Chapter 7) or multiple amplifiers for different gains may be provided, and the gain switching for amplifiers may be performed according to the input signal level, for example.
As described above, it is difficult to say that the type using a tri-color white LED is suitable from the viewpoints of costs and general versatility since the LED itself is expensive though the use of the tri-color white LED can increase the transmission rate.
On the other hand, in a case where a blue light-excited white LED is used, high speed data transmission can be performed by using the LED light transmission color filter 952 that blocks the light emitted from the phosphor 908 with a low speed of response as described above, without the problem of costs. However, the cutoff frequency of a general blue LED is only about several tens MHz (refer to “Kashikou Tsushin Yo LED Doraiba No Shisaku To Kashikou LED No Outou Seinou No Hyouka (Prototyping LED Driver for Visible Light Communication and Evaluating Response Performance of Visible Light LED)”, IEICE Technocal Report ICD 2005-44, Vol. 105, No. 184), and OOK modulation at a transmission rate over the frequency may also dull the output optical signal as shown in FIG. 7B, which may cause an intercede interference. Therefore, the upper limit of the data transmission rate is limited, and a transmission rate of about several tens Mbps can be only obtained. The data transmission quality also deteriorates.
Furthermore, like the case using the blue light-excited white LED, the problem of the transmission rate decreased by a low speed of response of light emitted by the phosphor may not be avoided in the data transmission using an ultraviolet light-excited white LED, and the driving voltage for the ultraviolet LED disadvantageously increases.
In addition, though the speed of response of light output from a phosphor may be increased by improving a phosphor material, there may be problems that a desired illumination cannot be obtained and/or that the cost of the phosphor material itself may increase. For those reasons, it is more advantageous to perform high speed data transmission by using a generic and inexpensive blue light-excited white LED.
On the other hand, there is no denying that the circuit structure is complicated in the method by which a larger dynamic range is obtained by using an AGC circuit or switching the gains of amplifiers according to the input signal level. In a case where the quantity of light entering to a PD is large since the distance between a transmitter and a receiver is short, the Space Charge Effect (refer to “Hikari Tsushin Soshi Kogaku (Optical Communication Device Engineering)”, Kougaku Tosho) causes a wave tail at the trailing edge of a signal waveform, which may cause an intercode interference. As a result, good transmission quality may not be obtained.